This invention relates to an electronic balance of the electromagnetic compensating type for the accurate measurement of forces, particularly for the measurement of forces generated in the weighing of masses. In particular, this invention relates to an improved electronic circuit arrangement including a more sensitive transducer sensing means and a bifurcated operating circuit which separates the force compensating function from the measuring function. The sensitivity of electromagnetic compensating balances is dependent on means to accurately sense minute displacements of a movable member relative to a reference point, circuit means to develop a defined compensating current, and means to accurately measure the electrical energy necessary to maintain the movable member at an equilibrium level. By improving the sensitivity of the transducer means to determine the location of the movable member and by making the means for compensating for the force to be measured independent of the measuring means, tolerance and error levels can be isolated and minimized.
Prior art electromagnetically compensated balances have reached a performance level which surpasses more conventional spring or knife edge balances in the accurate measuring of forces. Electromagnetically compensated balances utilize a permanent magnet circuit coupled to an electromagnetic circuit to generate a compensating force opposing the loading force on a movable member during a weighing operation. The arrangement is similar to a voicecoil actuator in that a coil wound bobbin is movable with reference to a stationary permanent magnet assembly. For use as a balance device, the coil is located with reference to the magnet assembly by a sensing means. Displacement of the coil by loading with a mass to be weighed is immediately sensed and inhibited by an appropriate d.c. current supplied to the coil to drive the coil to an equilibrium position. The weight of the mass is determined by measuring the d.c. current applied to the coil.
In prior art devices, the sensing means has comprised a pair of capacitor rings mounted with respect to the movable member on which the coil is mounted such that displacement in the movable member is reflected in changes in capacitance, one capacitor ring increasing in capacitance and the other decreasing. Because of the structural arrangement of the coil with respect to an annular air gap in the magnet assembly, the capacitor rings are necessarily located one above and one below the coil. The uniform spatial displacement compensates for error functions introduced by dimensional changes in the materials, occurring from temperature changes or other environmental or physical causes. However, because the movable member is suspended in the air gap and displaced from the magnet assembly, canting of the movable member may position one capacitor ring element closer to the reactive element on the movable member, thereby introducing an error producing inconsistency by the resulting change in capacitance. Additionally, local inconsistencies such as temperature gradients, bonding material and other factors affecting dielectric constants of one or the other capacitor rings are compensated by the paired arrangement.
Prior art methods of developing a compensating energy level in response to a sensed displacement have included circuits for generating compensating current pulses having a constant amplitude and a length that varies according to the extent of displacement of a movable member from its no-load position. The compensating current pulses are delivered to the coil and are concurrently measured by a high frequency counter pulse generator and a counter which counts during the length of the compensating current pulses.
The integral nature of the circuit means for developing the compensating current and the means for measuring the duration of the current pulses create an interdependence which is difficult to diagnose and correct when operating improperly. Such arrangements also require a specifically tailored counter means compatible to the compensation current means. These and other considerations have made the electromagnetic balance of this invention more versatile than prior art devices.